This invention relates to fuel supply systems for supplying volatile fuel at a controlled rate to an air-fuel mixing chamber leading to the subatmospheric intake manifold of an internal combustion engine and more particularly to such systems where a metering gear pump is used to supply fuel at a controlled rate and still more particularly where the gear pump is operated at elevated pressures and a low pressure differential.
It is well known to control fuel flow in an internal combustion engine, especially to maintain an appropriate air/fuel ratio, as is disclosed in Priegel U.S. Pat. No. 3,817,225, issued June 18, 1974 for "Electronic Carburetion System for Low Exhaust Emissions of Internal Combustion Engines." Priegel discloses a system wherein the rate of air flow and certain other parameters are measured and used to control the drive of a positive displacement metering pump to supply fuel at an appropriate air/fuel ratio.
It has also been suggested, as shown in Milam U.S. Pat. No. 3,643,635, issued Feb. 22, 1972 for "Electronic Fuel Injection System," that such controlled fuel supply systems utilize gear pumps for the fuel metering pumps. Gear pumps have not previously proven satisfactory for such purposes, however, as they have not provided proper response to the control signals. That is, they have not pumped fuel at the rate demanded by the control signals. It has now been discovered that inaccuracy is not inherent in metering gear pumps, and that the problems have arisen from two sources, gas bubbles in the system and leakage through the pump.
Gas bubbles passing through a metering pump displace liquid fuel. As metering pumps are volumetric, the rate of fuel flow is decreased when gas bubbles pass through the pump, making the air/fuel mixture leaner than demanded by the control signals. As the fuel used in internal combustion engines is highly volatile and as the engines operate at high temperatures, frequently ambient conditions produce hot spots in the fuel system that generate such bubbles, particularly at high ambient temperatures and low ambient pressures. (This is what causes the well known vapor lock in conventional fuel induction systems.) Such conditions also may produce cavitation at the pump impellers, similarly spoiling the metering capability of the pump. The problem is aggravated where the fuel is supplied to a subatmospheric mixing chamber, as is typical of internal combustion engines with intake manifolds pumped out by piston action.
Leakage is a problem because gear pumps and other rotary positive displacement pumps do not have positive seals in the pumping structure. The pumps therefore leak when there is any substantial pressure differential between the two sides of the pumps.
Both problems are present in systems like that of Milam. Milam utilizes a low pressure pump to supply fuel at low pressure to the intake side of a metering gear pump, thus producing conditions under which bubbles may be produced. At the same time Milam utilizes his gear pump to produce substantial pressure necessary to overcome the force of a biasing spring and to force fuel into the mixing chamber at proper velocity. Such pressure causes leakage of fuel back through the pump so that fuel flow is not linear with pump speed.
The problem of bubbles and cavitation can be minimized by supplying fuel to the metering pump at a relatively high pressure as assures that the fuel remains in its liquid phase. This, however, makes the problem of leakage worse, and the pumps leak fuel even when stationary. It has been suggested that the leakage problem can be reduced by keeping the pressure differential across the pump relatively constant so that leakage is constant and can be allowed for. See, for example, Meyer et al. U.S. Pat. No. 3,908,360, issued Sept. 30, 1975 for "Pump Metering Fuel Control System." However, any system requiring allowance to be made for leakage is inherently inferior to a measuring system where pump speed is proportional to fuel flow and can be taken directly as a measure of fuel flow. Also, this requires wasteful pumping just to offset the leakage loss.
It is therefore a primary object of the invention to provide a fuel supply system for supplying volatile fuel at a controlled rate to an air-fuel mixing chamber leading to a subatmospheric intake manifold of an internal combustion engine utilizing a metering rotary positive displacement pump, the output of which is proportionally related to rate of fuel flow, whereby a measure of pump speed may be fed back for use with a control signal to drive the pump at the desired rate. In accordance with the present invention, means are provided to supply the fuel to the inlet side of a metering gear pump at a substantial elevated pressure at which the fuel remains liquid passing through the gear pump. Whereas this would aggravate the leakage problem is the discharge side of the gear pump went directly to the subatmospheric pressure in the mixing chamber, the present invention provides an adaptation of the ancient flow control apparatus shown in Callan U.S. Pat. No. 1,272,212, issued July 9, 1918 for "Flow Controlling Apparatus," wherein leakage is precluded by equalization of the pressures on the two sides of the gear pump.
The Callan apparatus is for spinning filaments. The inlet pressure to the gear pump of Callan need only be sufficient to exceed the pressure required to deliver the desired quantity of fluid, as Callan states at page 1, lines 32 to 34. Further, there is no positive closure for Callan's equalizer valve, so that leakage from the valve is minimized only by the balance of pressures on the two sides of the diaphragm of the valve, as stated by Callan at page 2, lines 100 to 111. In gasoline engines, this is not good enough.
Thus, in accordance with the present invention, the equalizer valve includes a valve body forming a valve chamber with an outlet opening therein, a resilient diaphragm mounted in the chamber, and a valve closure member supported by the diaphragm for closing the outlet opening, with the diaphragm dividing the chamber in two parts and mounted for movement by pressure differentials between the two parts, one part being coupled to the inlet side of the gear pump and the other part being coupled to the discharge side. The diaphragm is biased for positive closure of the outlet opening by the valve closure member when the pressure on the discharge side is less than a small predetermined differential greater than the pressure on the inlet side, such small predetermined differential being insufficient to produce any substantial leakage through the gear pump. This assures positive closure when the gear pump is stopped, as when no fuel is demanded, so that fuel will not leak into the engine. However, unlike the operation of devices of Milam or Meyer et al., the pressure differential limit at which the valve is opened is so low that there is no substantial leakage through the gear pump, assuring proportionality of output as a function of speed.
A further object of the invention is to provide a fuel distribution system having a flow splitter for distributing fuel flow from a supply conduit to a plurality of fuel passages for separately directing the fuel to the mixing chamber.